The invention relates to an optical device comprising two substantially parallel substrates with electrode layers on their facing sides for switching an optically active layer which is present between said substrates. Furthermore the present invention relates to a hybrid layer for use in such an optical device.
Optical devices as mentioned in the opening paragraph are known as such, for instance, in the form of optical shutters or liquid crystal displays.
A severe drawback of the known optical devices is their relatively high switching voltage.
Recently introduced display devices based on cholesteric liquid crystals (CLCs) are still considered limited in capabilities. The major roadblock for realization of the benefits of chiral liquid crystal optical devices and displays has been difficulty in creating sufficient long range alignment of the helical structure in the preferred orientation. Alignment attempts to date have used surface alignment layers on either side of the cell containing the liquid crystal to produce strong alignment at the surfaces. Unfortunately, it has been difficult to control alignment within the bulk of the cell where the liquid crystal prefers to form domain structures separated by unaligned dislocation regions. To achieve good contrast, cell thickness must be increased yet as it is, alignment, and optical properties, degrade.
The present invention aims among others at providing optical devices having a relatively low switching voltage.
In addition, the present invention aims at providing optical devices having sufficient long range alignment of the helical structure and structural control in three dimensions.
This object is achieved in optical device comprising two substantially parallel substrates with electrode layers on their facing sides for switching an optically active layer which is present between said substrates, these optical devices in accordance with the present invention being characterized in that the optically active layer is formed as a hybrid layer containing an optically anisotropic material as well as a porous columnar structure. Wherever in this Application the wording xe2x80x9ccolumnar structurexe2x80x9d is mentioned, different kinds of structures are meant, which can be obtained by the method to be described. Such shapes are, for example, helical structures, pillars, slanted pillars, zig-zag, chiral or sinusoidal structures. Especially when using liquid crystal material for the anisotropic material, the presence of said structure results in a lowering of the switching voltage of the optical device.
Here we demonstrate a technique where a liquid crystal material is embedded in an inorganic porous backbone structure to produce strong alignment and structural control in three dimensions. Cell thickness is limited only by difficulties in fabrication of thick films, with 50 xcexcm thickness easily obtainable. In addition, liquid crystal alignment structures can be designed to engineer desired optical responses. For example, the narrow bandwidth of transmission/reflection typical of a CLC cell can be increased by producing a graded pitch structure. With the present method, pitch gradients or other structures can be accomplished with simple software modification to the deposition control system. The pitch can even be reversed within the layer, leading to substantially 100% reflection. Finally, because of the versatility of the present method, all polarization components needed for cell fabrication could be conducted with a small number of deposition steps, all based on the present method, to produce a complete device. While numerous particulars still remain to be investigated, the present method appears to be a promising technique for creating liquid crystal devices for display applications, but also for other components, both active and passive.
The invention also relates to a hybrid layer for use in an optical device, said layer containing an optically anisotropic material as well as a porous columnar structure. Said structure results in a lowering of the switching voltage of the optical device. Preferably the columnar structures comprise helical structures, in which the helixes may have, if wanted, square sides.
Chiral optical devices are used primarily for filtering of circularly polarized light, for example in liquid crystal (LC) displays. Various optical switching techniques based on chiral liquid crystals (CLCs) have been envisaged, with optical properties superior to linear polarization based devices, such as the twisted nematic cells used in the majority of commercial liquid crystal displays. In truth, twisted nematic cells are one type of chiral optical device where the chiral xe2x80x9ctwistxe2x80x9d length is considerably longer than the wavelength of visible light. The chiral optical devices discussed here have twists or pitches comparable to the wavelengths of visible light and operate within the xe2x80x9cresonancexe2x80x9d regime, corresponding to the xe2x80x9czone of selective reflectionxe2x80x9d in the CLC literature. Switching with a chiral optical device is based on the phenomenon of circular Bragg reflection, where one of the left- or right-circularly polarized light components is selectively reflected by the helical structure of the chiral material. Circular Bragg reflection arises from constructive scattering of circular polarized light from helical structures, and is fundamentally very similar to constructive interference reflections of plane polarized light from high/low index multilayers, Circular Bragg reflection allows light to be polarized for switching (in display and other photonic applications) without the use of absorbing polarizers, such as those used in linear polarization devices, which reduce power efficiency by absorbing half of the light available for transmission through the device.
The most commonly used chiral optical materials, chiral nematic liquid crystals (CLCs), are composed of nematic (rodlike) molecules with a small asymmetry in shape, or a mix of nematic molecules with an asymmetric additive. The structure of a layer of these molecules can be described as being composed of many sheets with all the rodlike molecules aligned within a sheet, but with a small rotation in orientation from one sheet to the next. The orientation rotates in a helical fashion through the cell, with one full molecular rotation called the pitch, p. Note: the first liquid crystal which displayed this orientation were closely associated with cholesterol and this phase was originally named the xe2x80x9ccholestericxe2x80x9d phase. The more accurate name for this phase is xe2x80x9cchiral nematicxe2x80x9d, however, and is the name that will be used in this description. The polarization selection property of CLCs, circular Brag reflection, occurs between xcexa31=pno and xcexa32=pne where no and ne are the ordinary and extraordinary refractive indices of the locally uniaxial structure. Within this reflection band, right-handed light is reflected from a right-handed helix, and left-handed light is transmitted. Alternatively, left-handed light is reflected from a left-handed helix, and right-handed light is transmitted. Wavelengths outside of the reflection band are transmitted in all polarizations.
The present invention aims at providing a film forming system method that allows for the growth of complex microstructures with predetermined patterns of growth. In addition porosity and optical properties of the shadow sculpted thin film are enhanced by expanding the range of incidence angles of the vapor flux.
Further, the inventors have found to their surprise that rotation of the substrate while it is exposed to an oblique incident vapor flux at polar angles greater than about 80.degree. produces well defined microstructures.
Therefore, in accordance with one aspect of the invention, there is provided a method of sculpting vapor deposited thin films, the method comprising the steps of:
initially exposing a surface of a substrate to a vapor flux at an oblique incident angle to grow a columnar thin film; and
subsequently, and while continuing to expose the surface to vapor flux, rotating the substrate about an axis parallel to the plane of the substrate.
In a further aspect of the invention, there includes the step of, while initially exposing the substrate to vapor flux, moving the substrate to alter the direction of growth of the columns, as for example by rotating the substrate about a normal to the surface of the substrate to create a helical film growth. The substrate may then be rotated about an axis parallel to the plane of the substrate to form a cap for the helical thin film growth. In a further aspect, a cap may be formed by exposing the substrate to a vapor flux in conditions of high diffusion length, such that a dense uniform mass is obtained, such as by heating the substrate to nearly the melting point of the material forming the vapor flux.
In a further aspect of the invention, tailored film growths may be obtained by (a) establishing, in a computer, a desired pattern of thin film growth; (b) while exposing a surface of a substrate to a vapor flux at an oblique incident angle, changing the orientation of the surface in relation to the angle of incidence of the vapor flux; (c) providing control signals to the computer indicative of thin film growth on the substrate; and (d) automatically controlling the rate of change of the orientation of the surface in response to the control signals to grow the thin film according to the desired pattern.
In a still further aspect of the invention, there is provided a method of sculpting vapor deposited thin films, the method comprising the steps of exposing a surface of a substrate to a vapor flux at an oblique incident angle; and, at the same time, rotating the substrate about a normal to the surface while maintaining the oblique angle at greater than 80.degree.
The desired pattern of film growth may also be tailored further. For example, a film may be started with a planar film of low porosity by rotating the substrate through exponentially increasing polar angles (zero to near 90.degree.), with rapid azimuthal rotation, and then columns may be grown on the substrate, with or without rotation. In one embodiment of a tailored growth pattern, the substrate is maintained at a constant polar angle while the substrate is repeatedly (a) rotated azimuthally a set number of degrees, for example 90.degree., and (b) held at a constant azimuthal position while the film grows obliquely, but linearly. The result is a helix with square sides in this instance. In general, the number of sides of the helix is 360/.gamma., where .gamma. is the number of the degrees the substrate is rotating during periods of deposition.
Since the deposition rate tends to vary considerably during deposition, to achieve helical growths with constant pitch, the rotation speed may need to be increased and/or decreased during deposition. In addition, by increasing/decreasing rotation relative to the deposition rate, helices with reduced/increased pitch, or helices with graded pitch may be obtained.
In general, the polar angle during an initial deposition period in which helical microstructures are to be produced should be greater than about 80.degree.